Seems it calls DoRun, which invokes either CFRunLoopRunInMode with kCFTimeIntervalMax, or [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
                             beforeDate:[NSDate distantFuture]];

i.e. both say "block until there is work available or until the "distant future" has been reached (i.e. forever).

I own too many bugs right now, so I'm Wontfixing - feel free to reopen.

Thanks! Reopening for a follow-up question.

I only see the call to CFRunLoopRunInMode(), not sure which second branch you're referring to.

You're right that the documentation specifies that the distant future should be reached if returnAfterSourceHandled==false, which is the case.

But then, why does the function have a loop? Why is kCFRunLoopRunFinished handled in https://cs.chromium.org/chromium/src/base/message_loop/message_pump_mac.mm?l=673&rcl=918863bc4660aa0cc992b6cb7d4611bb4809e6a6?

DoRun is a virtual method. The UI thread uses NSRunLoop. MessagePumpNSRunLoop::DoRun has a loop too - maybe that's a clue. The loops are probably there for a good reason. Do you think it's a bug? Sounds like you could do some research if you're interested - Apple's documentation probably has the answer. Perhaps start at https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Multithreading/RunLoopManagement/RunLoopManagement.html